Circuits for electric discharge lamps



Feb 13, 1951 J. CATES 2,541,033

CIRCUITS FOR ELECTRIC DISCHARGE LAMPS Filed Aug. 16, 1946 Fig. I. Fig.2.

H] S Aftorn ey.

Patented Feb. 13, 1951 CIRCUITS FOR ELECTRIC DISCHARGE LAMPS JuliusOates, Wembley, England, assignor to General Electric Company, acorporation of New York Application August 16, 1946, Serial No. 690,980In Great Britain July 2, 1945 2 Claims.

The present invention relates to circuits for electric discharge lamps,and is a modification of the invention disclosed and broadly claimed inapplication Serial No. 731,559, filed February 28, 1947, in the name ofHarold W. Lord and assigned to the same assignee, now abandoned.

It is often required to use capacitive circuits for operating dischargelamps since such circuits can be run in parallel with inductive circuitswith the results that the over-all power factor can be made nearly unityand the combined light from a lamp in a capacitive circuit and one in aninductive circuit exhibits less stroboscopic flicker than that fromeither lamp alone.

When a lamp in a capacitive circuit is operated directly from supplymains, the usual form of a ballast consists of a capacitor in serieswith a choke, the latter being essential to render the circuit stableand to make the wave form of the current approximately sinusoidal. Theimpedance of the capacitor is usually made about double that.of thechoke and during normal running of the lamp the voltage across thecapacitor is about double that across the choke, and also about doublethe supply voltage. If the impedance of the choke is made too small, thecurrent wave form becomes peaky, flicker may be noticeable and the lifeof the lamp may be adversely affected.

Where it is required to step-up the voltage applied to the lamp, it isadvantageous to use a stray-field (high leakage reactance) transformerwhich performs the functions of an ideal transformer and a series choke.There are obvious advantages in using a stray-field transformer with acapacitive circuit since a separate choke is then unnecessary.

Assuming that such an arrangement is used and that the open circuitvoltage across the output terminals of the transformer is V0, thevoltage across the capacitor and that across the output terminals willbe of the order of twice VD when the lamp is operating normally. Henceif the transformer is designed to have, on open circuit, a suitable fluxdensity in the iron associated with the primary and secondary coils (forexample, with silicon steel a peak value of about 10,000 lines persquare centimeter) then, when the lamp is operating normally, the fluxdensity in the iron associated with the secondary coil will reach asaturation value and give rise to objectionable flicker in the lamp. Theflux density in the iron associated with the primary coil remainsconstant since the voltage across this coil remains constant.

Such saturation can be avoided by sufficiently increasing the crosssection of the iron associated with the secondary coil, but the fluxdensity in this part of the iron will then be relatively very low onopen circuit and if the cross-section of the iron associated with theprimary winding were the same, as is usually convenient, the fluxdensity in this part of the iron would be unnecessarily low under allconditions. Such a transformer would clearly be large and expensive.

It is an object of the present invention to provide a capacitive circuitfor an electric discharge lamp, employing a stray-field transformer, inwhich the above-mentioned disadvantage is removed or substantiallyreduced.

According to the present invention, a capacitive circuit for an electricdischarge lamp comprises a stray-field transformer having a primary anda secondary winding arran ed upon a core of magnetic material. Amagnetic leakage path is provided for magnetic flux between the saidwindings, and the sa d core is provided with an air gap which lies inthe path of flux passing through the said secondary winding and whollyor largely on the side of the said leakage path remote from the primarywinding.

The effect of this air gap is to increase substantially the reluctanceof the flux path associated with the secondary winding. By a suitablechoice of the size of this air gap, and of the reluctance of the saidleakage path, the crosssection of the magnetic material associated withthe two windings can be made the same and such that the flux density inthe primary winding has a suitable, relatively high, value andnevertheless, the flux density in the secondary winding when the lamp isoperating normally may be arranged to have a suitable value belowsaturation.

The invention will be better understood from the following descriptiontakgen in connection with the accompanying drawing and its scope will bepointed out in the a pended claims.

In the drawing, Fig. 1 illustrates diagrammatically an embodiment of theinvention using a shell-type core with an auto-connected reactancetransformer, Figs..2, 3, 4 and 5 show modified core arrangements andwinding connections, and Fig. 6 illustrates a double reactanceautotransformer for energizing separate lead and lag discharge lampcircuits.

In Fig.1 a shell-type transformer I is used and the primary andsecondary windings 2 and 3 are arrayed upon the central limb 4 withreactance iron stampings 5, providing the leakage path, between them.Air or other non-magnetic gaps B are provided in the usual mannerbetween the ends of the reactance stampings 5 and the limbs l of thecore. In addition, an air or other non-magnetic gap 8 is providedbetween the central limb 4 of the core and the end limb 9 uniting thetwo outer limbs 1 at the end of the transformer at which the secondarywinding 3 is disposed. All the magnetic flux linking. the secondarywinding 3 thus passes through this air gap 8.

One such transformer has the following dimensions in inches. The outsidedimensions of the main core stampings are 5 5% in length (in a directionparallel to the axis of the central limb 4) and 4 in width and thethickness of the stack of stampings is 21%. The width of the side andend limbs 1 and 9 is and that of the central limb 4 is I The reactancestampings 5 on each side of the central limb 4 have a length equal tothe thickness of the main core, i. e. 2 1%, each stack is 1%,; thick andthe width of the stampings is -Z-, namely such that they extend from thecentral limb 4 to within 3% of the side limbs 'l'. The air gap 8 betweenthe central limb 4 and the end limb 9 at the secondary winding end is%4.

The primary winding 2 consists of 4'77 turns of 19 S. W. G. enamelledwire and the secondary winding 3 is of 552 turns of 20%; S. W. G. enamelled wire. The transformer is auto-connected.

This transformer is suitable for operating from a supply of 240 volts 50cycles a fluorescent, lowpressure, hot cathode, mercury vapor lamp ll!rated at 80 watts, of length 60 inches and diameter 1 inches. Acapacitor ll of 5.5 mfds. rated at 500 volts A. C. is used and a starterswitch I2 of known kind may be employed.

The reactance stampings 5 are preferably inserted in such a way thatthey extend over substantially the whole thickness of the main core. Ifthey are withdrawn unduly the wave form of the discharge currentdeteriorates and flicker is noticeable.

In a modification shown in Fig. 2 of the trans former above described,instead of the air gap 3 being between the central limb 4 and the endlimb 9 adiacent to the secondary winding 3, it may be disposed in thecentral limb 4 between one end of the secondary winding 3 and thereactance stampings 5. Instead of, or in addition to, the air gapsdescribed above, air gaps 8 may be provided in the side limbs 1 in thepath of the magnetic flux linking the secondary winding 3 and wholly orlargely on the side of the reactance stampings 5 remote from the primarywinding 2. This is shown in Fig. 3.

In another construction shown in Fig. 4, two E-shaped cores l3 and I4are used, one having the secondary winding 3 on its central limb and theother having the primary winding 2 on its central limb and the reactancestampings 5 between the extremity of the central limb and the outerlimbs. These two cores are then clamped together with the extremities oftheir limbs close together but separated by non-magnetic material toprovide the required air gaps 8.

In a further form of transformer, shown in Fig. 5, which may beemployed, two approximately U-shaped cores l5 and 16 are used, twosecondary windings 3 and 3' being preferably provided on the limbs ofone core and two primary windings 2 and 2 being preferably provided onthe limbs of the other core, one set of reactance stampings 5 beingdisposed between the limbs of the latter core l5 and spaced therefrom bysuitable air gaps 6. These two cores are clamped together with the limbsseparated by non-magnetic material of suitable thickness to provide airgaps 8 of the required lengths. The primary and secondary winding areconnected in series to provide an auto-wound transformer.

In a modification, shown in Fig. 6, of the transformer first describedabove, for use with two discharge lamps, one in a capacitive and theother in an inductive circuit, a single primary winding 2 is arranged onthe central limb 4 with a secondary winding 3 and 3" on each side of it.Reactance stampings 5 are provided between the primary coil 2 and eachsecondary coil 3 and 3" and an air gap 8 is provided between one end ofthe central. limb 4, and one end limb 9. The secondary coil 3 at thisend is connected to supply the capacitive circuit and that (3) at theother end supplies the inductive circuit. It may be found desirable, inorder to make the open circuit voltage substantially the same on bothsecc-ndary windings, to connect the latter secondary coil 3 to a tappingIT on the primary winding 2 rather than to one end thereof and toconnect the former secondary coil 3 to one end of the primary winding 2.The other forms of transformer described can also be modified to operatetwo lamps.

Instead of an auto-connected stray-field transformer, a double-wound,stray-field transformer can be used as shown in Figs. 2 and 4. Moreover,the invention is applicable both to hot cathode and cold cathodedischarge lamps.

The open circuit voltage across a secondary coil can be adjusted byaltering (1) the number. of turns on the secondary coil, (2) the pointon the primary winding to which the secondary coil is connected (in thecase of an auto-connected transformer), (3) the length of the air gapsor gaps in the main iron core, (4) the length of the air gaps or thedepth of iron in the reactance paths, or by a combination of two or moreof these steps.

It is also necessary that the secondary winding. associated with acapacitive circuit should have a high enough inductance since if theinductance is too low bad flicker may occur.

While there have been shown and described particular embodiments of thisinvention, it will be obvious to those skilled in the art that variouschanges and modifications can be made therein without departing from theinvention and, therefore, it is aimed in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In combination, a magnetic core, a primary winding and two secondarywindings on said core, said core including magnetic leakage flux pathsbetween said primary winding and said secondary windings, said coreincluding a high reluctance section between one secondary winding andthe leakage flux path which is between it and said primary winding, acapacitor, an electric discharge lamp connected in series with saidcapacitor across said last-mentioned secondary winding and said primarywinding in series, and a second electric discharge lamp connected inseries with the other secondary winding and a fraction of said primarywinding.

2. In combination, a magnetic core, a primary winding and two secondarywindings on said core, said core including magnetic leakage flux pathsbetween said primary winding and both of saidsecondary windings, saidcore including a high reluctance section between one secondary windingand the leakage path between it and said primary winding, saidlast-mentioned secondary Winding and said primary winding being seriallyconnected for energizing a leading current circuit for an electricdischarge lamp, the other secondary winding and a fraction of saidprimary winding being serially connected for op- 6 erating a laggingcurrent circuit for an electricdischarge lamp.

JULIUS GATES.

REFERENCES CITED Certificate of Correction Patent No. 2,541,033 February13, 1951 JULIUS GATES It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows:

Column 1, line 6, abandoned;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the casein the Patent Office.

Signed and sealed this 26th day of June, A. D. 1951.

after assignee strike out the comma and Words now! THOMAS F. MURPHY,

Assistant Oomtm'sst'omr of Patents.

Certificate of Correction Patent N 0. 2,541,033 February 13, 1951 JULIUSGATES It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correctlon asfollows:

Column 1, line 6, after assignee strike out the comma and words nowabandoned;

and that the said Letters Patent should be read as corrected above, sothat the same may conform to the record of the casein the Patent Oflice.

Signed and sealed this 26th day of June, A. D. 1951.

THOMAS F. MURPHY,

Assistant Gammz'ssz'oner of Patents.

